NBT_SessionService.py:  Overhaul message parsing.  Update everything.
NBT_SessionService.py:  Overhaul message parsing.  Update everything.

  * Added a generic dump() function to pretty-print the messages.
  * Added more doctests.
  * Refuddlement of several comments.
  * Internal lookup tables map codes to descriptive strings.
    * Functions provide access to those mappings.
  * Rewrote the ParseMsg() function to make it more consistent.
    * Improved error checking.

manuella/Carnaval,ubiqx-org/Carnaval